Many portable wireless transmitting devices are used today in close proximity to the body, and are generally designed to meet electromagnetic radiation requirements set forth by the Federal Communications Commission (FCC) in the United States, and other similar government regulatory bodies abroad. Portable devices are getting smaller and more powerful, and are being equipped with multiple radios, many of them operating simultaneously, some of them transmitting simultaneously. Each transmitting radio exposes the user to some level of electromagnetic radiation. The acceptable radiation levels have not increased as the device power has increased. As a result, there is a need to design methods of transmission that control and reduce user exposure to electromagnetic radiation.
For example, cellular phones, especially so-called “smartphones”, are generally equipped with transmitters that operate in the 3G bands, including 800 MHz and 1900 MHz bands, the 4G bands, which can range from 700 MHz to 2700 MHz, and the WiFi band (2401-2483 MHz). These transmitters may run concurrently, depending on the mode of operation of the device.
More recently, there has been interest in millimeter-wave frequencies for mobile and portable communications. With millimeter waves, exposure to electromagnetic radiation will also be a concern, especially for devices that are held near the human body.
In testing a device for radiation levels, the FCC requires the transmitting device to be put into its mode of operation that yields the greatest exposure level. With multiple radios transmitting concurrently, the radiation is potentially additive. Hence, every added radio in a device places an additional compliance burden on each radio separately to meet radiation thresholds. The lower the radiation level of a given radio, the more likely the device as a whole will meet regulatory compliance. Generally, compliance is desired without having to sacrifice the output power of the individual radios.